The present invention relates to switches for use in inductive energy storage systems. More particularly, the present invention relates to switches providing opening or transfer switching action in energy transfer from an inductive energy stored source.
An inductive energy storage system includes a primary energy source, an inductor, and a primary switch element. A generalized diagram of an inductive store energy transfer circuit is shown in FIG. 1. The function of the switch in the circuit is to establish a sufficient voltage across the load terminals to transfer the current out of the switch and into the load. Any number of fundamentally different mechanisms may be used to accomplish the function. These include: varying the resistance of the switch element to establish an IR voltage drop across the load terminals, changing the inductance of the switch element to create an I dL/dt voltage across the load terminals, or a combination thereof.
Inductive energy storage systems are finding use as the power source in railguns. In an inductive energy storage system for such purpose, the primary energy source is typically a homopolar generator. A diagram of a simple homopolar generator (HPG) powered railgun circuit is shown in FIG. 2. The accelerating force in a parallel railgun accelerator is obtained by the interaction of the current in the driven armature with the magnetic field produced by the current in the rails. The switching requirements are especially severe.
In operation, the homopolar generator is motored up to speed and then switch S.sub.1 is closed discharging the HPG into the inductor L through switch S.sub.2. The current in the inductor rises to a peak in 0.1. to 0.5 seconds at which time switch S.sub.2 having carried in excess of 10.sup.5 coulombs must open, thereby transfering the current into the railgun. The current vaporizes the fuse creating an arc which accelerates the projectile by the Lorentz force.
Additional switching performance requirements are present in injected or distributed energy store railguns, because the opening of the switch S.sub.2 must be synchronized with the position of the moving projectile. Also, because the fuse is in parallel with switch S.sub.2 during charging of the inductor, current flows through the fuse producing heating action therein and possibly premature motion thereof.
A more desirable, but more difficult switching function is adopted in the railgun circuit diagram in FIG. 3. The operation of this circuit is similar to that shown in FIG. 2 except that when peak current is reached in the inductor, switch S.sub.2 is switched from position A to position B. This switching action introduces the fuse into the active circuit.
An additional requirement in practical railgun realization is that the energy transfer switch must be capable of dissipating heat generated therein, and should be capable of repetitive operation with only minor maintenance between shots. The seriousness of the heating problem is brought into focus when it is noted that in typical railgun operation a peak inductor current of 10.sup.6 amperes is transferred in 5.times.10.sup.-4 seconds at 1,000 volts, the energy dissipated in the switch is approximately 2.5.times.10.sup.5 Joules.